Variable-speed drives (VSD)—also known as frequency converters and as inverter drives, variable-frequency drives (VFD), adjustable-speed drives (ASD), or adjustable-frequency drives (AFD)—provide an efficient means of driving electrical motors, typically achieving efficiencies of 95% or more. In VSD-fed motors, the minimal phase-displacement between voltage and current minimises reactive power requirements and associated losses. The ability to operate such motors at low speeds allows a gradual start/stop to minimise starting currents and harmful transient mechanical stresses. In pumps and compressors, the low running speeds achievable can provide very large energy savings because the power consumption varies with the cube of the operating speed.
Many large, high-voltage motors used in the oil and gas industry are VSD-driven. Often these machines are located in explosion-risk environments. In order to maintain availability of these critical machines, on-line condition monitoring technology is required. However, whilst vibration and mechanical condition monitoring technologies for these motors are well established, reliable on-line partial discharge (OLPD) monitoring to assess localised damage in the motor stator winding insulation has been more difficult to achieve. Moreover, the transient voltage spikes associated with each current pulse from the VSD inverter can present an increased risk of insulation failure and/or motor bearing erosion due to the electrical discharges that may result. Thus, there is a clear and particular need for reliable on-line partial discharge monitoring of VSD-operated motors.
However, this presents several challenges. For example, it can be difficult to distinguish partial discharge pulses from background electromagnetic interference produced by the high speed switching of the VSD's power electronics, especially since the magnitudes of switching noise transients can often dwarf those of the partial discharge pulses of interest, sometimes by a factor of ten or more. The location of many large inverter-driven motors in confined and/or hazardous locations, such as explosion-risk (Ex/ATEX) zones, can severely limit the amount of additional instrumentation that could be installed for capturing or filtering partial discharge monitoring data.
Objects of the invention therefore include the provision of a partial discharge monitoring apparatus and method that overcomes the problems outlined above; the provision of reliable hardware and/or software technologies for online partial discharge monitoring of large VSD-fed high-voltage rotating machines; and/or improved means for discriminating partial discharge activity from VSD switching noise, preferably with sufficient precision that false positives where switching interference is incorrectly identified as partial discharge activity can be avoided.